The field of the invention is the biological treatment of wastewater, and particularly, instruments for measuring the amount of dissolved oxygen in the wastewater being treated.
The concentration of oxygen dissolved in wastewater is an important parameter in industrial and municipal treatment plants. Microorganisms are used to metabolize organic matter found in the wastewater and oxygen is required by the microorganisms to carry out this aerobic metabolic process. Insufficient oxygen either kills the microorganisms or creates a septic, anaerobic condition.
To control the amount of dissolved oxygen in wastewater accurate sensing instruments must be provided. A number of probes for measuring the amount of dissolved oxygen in a liquid are known and are commercially available. In all cases, however, the probes are submerged in the liquid and the dissolved molecular oxygen is reduced at a negatively charged electrode to generate an electric current directly proportional to the oxygen content. To measure dissolved oxygen in wastewater, therefore, the organic contaminants must be separated from the sensing element to enable the electrolytic process to proceed. This is accomplished by employing a selectively permeable membrane which allows the liquid to enter the electrolytic sensing cell when the probe is submerged, but which prevents the passage of contaminants.
Such prior dissolved oxygen measurement instruments have not operated successfully in wastewater with a heavy concentration of solids. The permeable membrane clogs repeatedly and is often torn by solids thus creating a serious maintenance problem.